The present invention relates to an electroluminescent device (ELD); and, more particularly, to a thin-film ELD having a thin-film current control layer.
In general, an ELD is designated to the device using a phenomenon of light emission when an electric field is applied upon material. Such ELD is largely classified into an organic ELD and an inorganic ELD according that the material provided as a phosphor layer is organic or inorganic material. The inorganic ELD is also classified into a thin film and a thick film type according to a thickness of phosphor layer.
Particularly, the thin-film ELD includes an alternate current (AC) and a direct current (DC) driving type whether an applied electric field has a polarity or not.
The AC thin-film ELD having two dielectric layers on upper and lower parts of a phosphor layer, has a long lifetime and a stabilized operation in comparison with the DC type, thus AC thin-film ELD has been applied to a display device which requires an endurance and a high resolution. However, the AC thin-film ELD requires high operating voltage of about 150xcx9c250 VP, therefore, an expensive driving circuit is needed to operate the AC thin-film ELD.
Meantime, since the DC thin film ELD has a merit of a low operation voltage, much attention has paid to develop a device with a high luminous efficiency and a good reliability by appropriately controlling electron supply into the phosphor layer.
An initial DC type ELD was a stacking structure of transparent electrodes, a thick-film phosphor layer, and metal electrodes in order. This device was required a forming process in order to make an operation of such thick ELD stable. During the forming process, much current flows across a device, and the phosphor layer is aged to a stabilized state. In a case of ZnS:Mn,Cu conductive thick-film phosphor, non-conductive ZnS:Mn layer with a thickness of about 1 xcexcm between a transparent electrode and the ZnS:Mn,Cu layer is formed through the forming process. In order to eliminate the forming process which causes an inconvenience and a difficulty in its control, the U.S. Pat. No. 4,859,904 disclosed a DC type ELD based on a thin-film/powder hybrid structure. The structure of this device consists of a transparent substrate, transparent electrodes, a thin-film phosphor layer, a thick-film current limiting layer, and metal electrodes in order. The thick-film current limiting layer was provided through a use of MnO2 powder. In this device, a thin-film phosphor and MnO2 powder layers work the same function of the nonconductive ZnS:Mn layer and conductive thick ZnS:Mn,Cu layer above, respectively.
In such ELD with the thin-film/powder hybrid structure, a contrast ratio of the device can be improved and a reduction of luminescence can be prevented by inserting a black color layer between the thin-film phosphor layer and the thick film current limiting layer.
With reference to FIG. 1, the U.S. Pat. No. 5,229,628 proposed an advanced DC-type hybrid ELD with a better reliability and a higher luminous efficiency than those of the thin-film/powder hybrid structure above. FIG. 1 illustrates an new DC-type thin-film/powder hybrid ELD with power supply 18 which has a stacking structure of a transparent substrate 11, transparent electrodes 12, a thin-film phosphor layer 13, a thin-film insertion layer 14, a thick-film current-limiting layer 15, and metal electrodes 17. As one of its embodiments, they used the thin-film phosphor layer 13 of 1 xcexcm ZnS:Mn, the thin-film insertion layer 14 of 0.1 xcexcm ZnSe, and the thick-film current limiting layer 15 of 15 xcexcm MnO2. This device demonstrated an efficient of 0.80 lm/W and a lifetime of 20,000 hours or over. The insertion layer 14 acts as an energy barrier between the thin-film phosphor layer 13 and the thick-film current limiting layer 15, therefore the insertion layer 14 provides energetic electrons into the phosphor layer 13 by a field-assistant injection. The DC type thin-film/powder hybrid ELD with the insertion layer 14 provides a brightness and an efficiency increase of 1.5 times or over in comparison with a case of non-insertion. However, the DC thin-film/powder hybrid ELD shown in FIG. 1 has some shortcomings such that it is difficult to embody a flat display panel with a high resolution due to its thick-film characteristics and a luminous efficiency is lower than that of the conventional AC thin-film ELD.
Though not shown in the drawings, the U.S. Pat. No. 5,796,120 as another conventional technique proposed a tunnel type thin-film ELD. The tunnel type thin-film ELD was constructed by a stacking structure of a substrate, bottom electrodes, a lower thin-film energy barrier layer, a thin-film phosphor layer, an upper thin-film energy barrier layer and upper electrodes. In this tunnel thin-film ELD, when the electric field is applied the device, the electrons supplied from the metal electrode enter into the phosphor layer by tunneling the barrier layer such as CaF2 thin-film with a thickness of 5 nm and below. This device demonstrated to be operated at low voltage and to be able to control the luminescent characteristics with the applied filed and the barrier layer.
Practically, it is very difficult to deposit the very thin energy barrier layer less than 5 nm with a good thickness uniformity and a lattice-matched epitaxial growth. This kind of growth is only possible by a molecular beam epitaxy (MBE) method in a case that lattice constants between the energy barrier layer and the phosphor layer (or bottom electrode) coincide well. In addition, the thickness uniformity is very important factors to ensure a reliability of the device and to control a quantity and an energy of electrons tunneled into the phosphor layer.
Therefore, it is an object of the present invention to provide a thin-film ELD capable of driving at a low voltage and increasing a luminous efficiency and a resolution.
To achieve these and other advantages, and in accordance with the purpose of the present invention, in the thin-film ELD based on a stacking structure of a transparent substrate, transparent electrodes, a thin-film phosphor layer, a current control layer and metal electrodes in order, the ELD is constructed by a characteristic that the thin-film current control layer acts as an energy barrier layer, which supplies energetic electrons into the phosphor layer by a field-assistant injection of electron, and a current-limiting layer which prevents an electric field breakdown of the device caused by an excess current flow.
Further, in the inventive thin-film ELD comprising of a stacking of a transparent substrate, transparent electrodes, a thin-film phosphor layer, an energy barrier layer, a current-limiting layer and metal electrodes in order, it is characterized with that the energy barrier layer supplies energetic electrons into the phosphor layer by a field-assistant injection of electron, and the current-limiting layer prevents an electric field breakdown of the device caused by an excess current flow.